Recently, significant advances have been made in the development of solid-state light emitting devices. Among the most recent discoveries was the discovery that conjugated polymers are particularly well suited for this purpose in that they provide excellent charge transport characteristics and useful quantum efficiencies for luminescence. The most popular of the materials suitable for this use is poly (p-phenylene vinylene) (PPV) and its derivatives which are capable of being prepared in the form of a high quality film which evidences strong photoluminescence in a band centered near 2.5 eV.
Heretofore, synthesis of poly (p-phenylene vinylene) and some of its analogs has been commonly effected by preparing a precursor polymer and thermally converting the precursor in a vacuum or inert atmosphere to the desired poly (p-phenylene vinylene). A typical procedure for attaining this end, commonly called the Wessling route, involves polymerization of a bis sulfonium salt intermediate, prepared from dichloro p-xylene and tetrahydrothiophene, in a water-methanol mixture in the presence of a base and subsequently dialyzing against distilled water to remove inorganic salts and unreacted monomer. The precursor polymer is then recovered, dissolved in methanol and spin coated upon a suitable substrate. At this juncture, the precursor is thermally converted at a temperature ranging from 150.degree.-300.degree. C. in a vacuum or in an inert atmosphere comprising argon or nitrogen to yield the desired poly (p-phenylene vinylene).
In this process, the polymerization is divided into two steps, the first involving the formation of an anion by an acid-base equilibrium. In the second step, the anion undergoes a elimination reaction of tetrahydrothiophene to yield a quinone structure which polymerizes to a water soluble precursor polymer. In effecting this reaction, a group polarizer is employed to stabilize the anion and in the 1,6-elimination a leaving group is employed to form the quinoid. P-xylene derivatives possessing such polarizer and leaving group may be polymerized and the elimination of the polarizer results in the formation of poly(p-phenylene vinylene), PPV.
In this technique, the precursor polymer so formed is a polyelectrolyte which is soluble in only water and methanol, both of which are poor spinning solvents. Accordingly, the use of this technique has not proven satisfactory for the generation of films having high quality due to the formation of a large number of pinholes which typically lead to shorting of light emitting diode devices. Additionally, hydroxyl units in the precursor result in the formation of carbonyl groups in the final product which tend to lower photoluminescence and electroluminescence. Although efforts to reduce carbonyl formation in this processing sequence have been successful by conducting the conversion step in a hydrogen/nitrogen atmosphere, it is still advantageous to avoid the presence of hydroxyl groups and to be able to employ a precursor which is soluble in organic solvents.
Subsequent efforts to overcome the limitations of the Wessling technique involved the polymerization of .alpha.-chloro-.alpha.'-alkyl(aryl)sulfinyl- and .alpha.-chloro-.alpha.'-alkyl(aryl)sulfonyl-p-xylenes in a modified Wessling bissulfonium salt technique. Using a sulfinyl group polarizer, the resultant polymer is found to be soluble in organic solvents such as tricholorethylene, tetrahydrofuran and dimethylformaldehyde and is stable and may be converted to trans-PPV. Additionally, the use of a polarizer having a sulfonyl group results in a polymer of high thermal stability. And lastly, weight loss during conversion approaches 54% and tends to cause a diminution in film quality.
The use of sulfinyl or sulfonyl as a leaving group of the precursor polymer has inherent limitations. Thus, for example, the use of a sulfinyl group yields a precursor polymer having a molecular weight of only a few thousand daltons which is not high enough to obtain stable films. The use of the sulfonyl group permits the attainment of molecular weights ranging up to 30,000 daltons. However, such values are still significantly below the level of the precursor polymers prepared by the Wessling technique. Other limitations include the need for a conversion temperature close to 280.degree. C. and the two step synthesis of the monomer includes assymetrical coupling which can reduce the yield of the reaction below 50%.
Accordingly, workers in the art have continued to focus their interest upon the preparation of phenylene/vinylene films by novel procedures which obviate the foregoing limitations.